The functional significance of specific prolactin binding sites on ependyma of the choroid plexus will be investigated with emphasis towards 1)prolactin regulation of electrolyte balance between blood and cerebral-spinal fluid (CSF), and 2) receptors for an uptake and transport mechanisms for prolactin from blood into CSF. Electrolyte concentrations in serum and CSF of rats will be determined during hyperprolactinemic and hypoprolactinemic states and results compared to control values. Hyperprolactinemic will be achieved by subcutaneous injection of prolactin-secreting pituitary tumor cells. Hypoprolactinemia will be attained by hypophysectomy and daily intraperitoneal injections of bromocryptine, a potent inhibitor of prolactin secretion. An electron microscopic autoradiographic analysis of ovine 125-I-prolactin movement relative to ependyma cytology will assist in determining whether choroid ependyma are the final target cells or components of a blood to CSF transport mechanism for prolactin. Specific prolactin binding sites in the central nervous system (CNS) will be regionally mapped by application in vivo of the principles of the competitive-binding assay in conjunction with light and electron microscopic autoradiography. Following cerebral ventricular and vascular injections of ovine 125-I-prolactin and control unlabelled compounds, cell populations possessing specific prolactin binding sites will be ultra-structurally characterized.